In web-processing printing presses, in particular offset presses, whether they process one web substrate or a plurality of web substrates, given an elastic deformation of the web substrate, there is a linear relationship between the web tension and the web elongation or web stretch (deformation). The web tension of a specific web substrate is generally also dependent on the processing parameters of the printing press and of the printing substrate. For example, the type of rubber blankets used, the number of print units in thrown-on impression mode, and the type of printing substrate used, in particular paper, must be specified or considered. In practice, the web tension for infeeding the web substrate during operation of the printing press is typically varied in reaction to the actually occurring web tension in the superstructure upstream of a folding apparatus, until a desired or specified web tension is reached: when the web substrate is too slack, the web tension is increased; when the web substrate is stretched too tightly, the web tension is reduced. In this context, the decision whether to increase or reduce the web tension is often made by the printer or the machine operator and is, therefore, largely based on his/her experience. Setting the web tension to a correct value is vitally important to a proper functioning of the printing press.
A method and a device for controlling the elongation of a moving web substrate are known from U.S. Pat. No. 5,709,331. From a length change or a speed change occurring at a drive roller, the current web elongation is calculated, and when this current web elongation deviates from a desired elongation, the web tension is corrected to the desired value. In the process, there is no need to know the exact value of the elasticity modulus of the web substrate, since the relative deviation and thus a necessary correction can be directly ascertained. The method can be implemented using a web tensioning device of an unwind unit (or of reel changer). The web tension can be varied by using a dancer roller or web tension roller, which exerts a force on the web substrate.